Implantable cardiac devices are well known in the art. They may take the form of implantable defibrillators or cardioverters which treat accelerated rhythms of the heart such as fibrillation or implantable pacemakers which maintain the heart rate above a prescribed limit, such as, for example, to treat a bradycardia. Implantable cardiac devices are also known which incorporate both a pacemaker and a defibrillator.
A pacemaker may be considered as a pacing system. The pacing system is comprised of two major components. One component is a pulse generator which generates the pacing stimulation pulses and includes the electronic circuitry and the power cell or battery. The other component is the lead, or leads, having electrodes which electrically couple the pacemaker to the heart. A lead may provide both unipolar and bipolar pacing and/or sensing electrode configurations. In the unipolar configuration, the pacing stimulation pulses are applied or evoked responses are sensed between a single electrode carried by the lead, in electrical contact with the desired heart chamber, and the pulse generator case. The electrode serves as the cathode (negative pole) and the case serves as the anode (positive pole). In the bipolar configuration, the pacing stimulation pulses are applied or evoked responses are sensed between a pair of closely spaced electrodes carried by the lead, in electrical contact with the desired heart chamber, one electrode serving as the anode and the other electrode serving as the cathode.
Pacemakers deliver pacing pulses to the heart to cause the stimulated heart chamber to contract when the patient's own intrinsic rhythm fails. To this end, pacemakers include sensing circuits that sense cardiac activity for the detection of intrinsic cardiac events such as intrinsic atrial events (P waves) and intrinsic ventricular events (R waves). By monitoring such P waves and/or R waves, the pacemaker circuits are able to determine the intrinsic rhythm of the heart and provide stimulation pacing pulses that force atrial and/or ventricular depolarizations at appropriate times in the cardiac cycle when required to help stabilize the electrical rhythm of the heart.
Implantable cardiac defibrillators (ICD's) are also well known in the art. These devices generally include an arrhythmia detector that detects accelerated arrhythmias, such as tachycardia or fibrillation. When such a tachyarrhythmia is detected, a pulse generator delivers electrical therapy to the patient's heart. A therapy for tachycardia may be anti-tachycardia pacing and a therapy for fibrillation may be a defibrillating shock. Such therapies are well known.
During atrial tachycardia (AT), the atria of the heart beat rapidly at an abnormally high rate. This can cause the ventricles to in turn beat at a high rate. Cardiac output may be reduced. The patient may experience dizziness or feel fatigued. Although not immediately life threatening, it is generally unpleasant to a patient.
Atrial fibrillation (AF) is a common atrial tachyarrhythmia and can occur suddenly. It results in rapid and chaotic activity of the atria of the heart. The chaotic atrial activity in turn causes the ventricular activity to become rapid and variable. Although not life threatening, it is associated with strokes thought to be caused by blood clots forming in areas of stagnant blood flow as a result of prolonged atrial fibrillation. In addition to strokes, symptoms of atrial fibrillation may include fatigue, syncope, congestive heart failure, weakness and dizziness.
Implantable cardiac device therapies are known which assist in precluding the occurrence of atrial tachyarrhythmia episodes. One therapy deals with pacing the atria at a rate slightly faster than the normal intrinsic rate. This is believed to maintain capture of the atria and more organized cardiac behavior, thus preventing an accelerated atrial arrhythmia from occurring. Unfortunately, this atrial pacing at a slightly faster atrial rate than normal can become annoying to a patient.
Antitachycardia pacing of the atria is also known to treat atrial tachycardia (AT) and assist in preventing an acceleration of the atrial rate into fibrillation. Here again the atria are paced at a rate above the normal intrinsic rate. It has been found to be most effective for more organized accelerated atrial rates corresponding to, for example, mean atrial cycle lengths above about 250 mS.
Less organized accelerated atrial rates corresponding to mean atrial cycle lengths less than 250 mS are generally best treated with a low voltage atrial cardioversion shock. When necessary, a higher voltage atrial defibrillation shock may be appropriate.
Obviously, no single therapy is always the appropriate therapy. At times, preventive therapy may be desirable while at other times termination therapy may be appropriate.
Because of the wide range of available accelerated atrial rhythm therapies, it would be helpful to have a way to make an appropriate therapy selection best suited for an individual patient. Recently, it has been observed that accelerated atrial arrhythmias may have a circadian quality. This may be used to advantage in predicting which of the many therapies should be employed at a given time. The present invention addresses this and other issues.